Grazie per le risposte e per il link cassano, comunque sono riuscito ad esportare tutti i dati al completo in Excel seguendo un'altra strada, questa per la precisione:

protected void Button1_Click(object sender, EventArgs e)
{
string conn = ConfigurationManager.AppSettings["ConnectionString"];
OracleConnection connessione = new OracleConnection(conn);
String comm = ViewState["query"].ToString();
OracleDataAdapter command = new OracleDataAdapter(comm, connessione);
DataSet dataset = new DataSet();
command.Fill(dataset);
connessione.Close();
if (dataset.Tables[0].Rows.Count > 0)
{
Response.Clear();
Response.AddHeader("content-disposition", "attachment;filename=File.xls");
Response.Charset = string.Empty;
Response.Cache.SetCacheability(HttpCacheability.No Cache);
Response.ContentType = "application/vnd.xls";
System.IO.StringWriter stringWrite = new System.IO.StringWriter();
System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);
System.Text.StringBuilder sb = new System.Text.StringBuilder();
sb.Append("<table>");
foreach (DataRow r in dataset.Tables[0].Rows)
{
sb.Append("<tr>");
foreach (DataColumn col in dataset.Tables[0].Columns)
sb.Append("<td>" + r[col].ToString() + "</td>");
sb.Append("</tr>");
}
sb.Append("</table>");
htmlWrite.Write(sb);
Response.Write(stringWrite.ToString());
Response.End();
}
}